Protein synthesis is required for a cell to recover from radiation- induced G2-arrest. It is the purpose of this investigation to determine the nature and functions of the protein(s) involved. Two lines of inquiry will be followed. The first is prompted by the observation that an arrested cell can be brought into a mitosis- like state by fusion with a mitotic cell. This process is believed to be mediated by the donation of a group of polypeptides termed Maturation Promoting Factors (MPF), thus it is suggested that recovery from radiation-induced G2-arrest may require MPF synthesis. The second approach is prompted by the observation that arrested cells may be brought into mitosis without protein synthesis by treatment with caffeine and cycloheximide. This suggests that the components of the progression "machinery" are actually intact in arrested cells (including MPF), but are inactivated. The proteins normally required for recovery must then be produced for some other activity, not directly related to progression, and probably unique to, overly abundant in, or characterized by enhanced synthesis in irradiated cells compared to age-equivalent controls. MPF will be isolated from arrested cells obtained by centrifugal elutriation and assayed for activity in Xenopus laevis oocytes. Protein synthesis in arrested cells will be monitored by incorporation of isotope-labelled amino acids, then the cells retrieved from arrest as a discrete population by caffeine treatment and mitotic cell selection. Specific polypeptides will be identified by 2-D gel electrophoresis. The work proposed is intended to determine the requirements for progression past the X-ray transition point. Such information may help us to understand why cell sensitivity to killing by irradiation increases so markedly at the X-ray transition point and may provide the means to manipulate both cell progression and cell radiosensitivity, both factors of importance to the treatment of cancer by radiation.